Old Bulgarian is an early Slavic language that was used in the First Bulgarian Empire during the 9th to 11th centuries. It is also known as Old Church Slavonic, as it was the first Slavic language to be used in religious texts. The language was developed by the brothers Cyril and Methodius, who created the Glagolitic alphabet to translate the Bible and other Christian texts. Old Bulgarian played a crucial role in the cultural and religious development of the Slavic peoples. It served as a liturgical language for the Orthodox Church and influenced the development of later Slavic languages, including Bulgarian, Serbian, and Russian.